Being able to write C# instead of JS is a pretty big point IMHO.

What do you gain in using this that you could not already do (better) on .net?

You don't gain anything what you lose is a bunch of overhead. It's the same advantages/disadvantages you get from sinatra vs. rails.

.net is incredibly well optimised for web apps, with built-in connection pooling and countless other goodies. For anything non-trivial I'll bet that a normal .net application will blow this out of the water in terms of performance.

It's not async and it is quite heavyweight. Your claim that ".net is incredibly well optimized" is true only for certain definitions.

The output of a really simple ASP.NET MVC hello-world app is measured in a couple of hundred requests per second without even touching the database. If you do the same on top of Node.js or Netty or Twisted, you're speaking about tens or even hundreds of thousands of requests per second.

     For anything non-trivial I'll bet that a normal .net
     application will blow this out of the water in terms of 
     performance.

That's not true, not even if you're comparing ASP.NET with normal web frameworks, such as Ruby on Rails.

You can get a lot of performance out of ASP.NET, but to do that you've got to rely on pretty ugly optimizations. To get a taste of what I'm talking about, check out this slightly old, but still relevant presentation: http://blog.whiletrue.com/2009/04/aspnet-mvc-performance/

TL;DR the author went from 5.9 requests / second to 390 requests / second, but to do that they had to cut out many nice features of ASP.NET MVC that used runtime introspection and they also added caching, something which you do anyway in any other web framework available.

Again, for some applications you can go fully async and speak about tens or hundreds of thousands of requests per second served by the same server.

Out of interest, do you know if the people at Stack Overflow had these issues?

If you read the errata of that post you'll see that the author could have got to 390 requests a second by turning off debug mode. No ugly optimisations necessary.

I have not looked at this framework in particular sorry, I was only really commenting on JS vs. C# here.

I recently came across SignalR, it's a very well written and simple framework to use.

The buzzwords are web sockets, comet, ajax push, etc.